We modeled the trajectories with growth curve models. A limitation of growth curve models is that they assume that all individuals can be described by the same parameters of change (e.g., everyone shows a quadratic trajectory; Connell & Frye, 2006). Growth curve models do not assume, however, that individuals’ change is homogeneous. In the present study, the quadratic form was allowed to vary across all individuals, allowing each individual to have a different trajectory (with different intercepts, slopes, and curvatures). This form of describing trajectories is in contrast to previous studies that have examined trajectories of subgroups of people. For example, similar to the developmental taxonomy proposed by Moffitt (1993), Odgers et al. (2008) identified four different subgroups that followed different trajectories: life-course persistent, adolescent-onset, childhood-limited, and low. At the very least, subgroup modeling can be useful as a heuristic of some general patterns of externalizing trajectories and simplified characterizations of continuous trajectories. An assumption in modeling subgroups’ trajectories, however, is that all individuals within a subgroup follow a similar (though not necessarily the same) trajectory, and yet qualitatively different
